The present invention relates to an electrostatic recording system, and more particularly to a development apparatus for developing a latent electrostatic image formed on a photo-sensitive drum into a visual image by making a developer agent adhere to the latent electrostatic image.
The apparatuses for developing a latent electrostatic image on a photo-sensitive drum in an electrostatic recording system or an electronic photographic system are generally grouped into two types. One is a dry type development apparatus and the other is a wet type development apparatus.
In the dry type development apparatus powder-state ink or toner is used for developing a latent electrostatic image. The dry type development apparatus is further classified into a two-constituent and a single-constituent development apparatus. In the two-constituent development apparatus, a constant proportion mixture of toner and carrier consisting of magnetic powder is used as a developer agent. The toner is charged by the carrier and made to adhere to the latent electrostatic image on the photo-sensitive drum. Since this development apparatus can provide a high quality visual image, it is most generally employed in a copying device, an electrostatic recording printer, or the like, and therefore, the apparatus has been technically well developed. However, the apparatus requires control means for maintaining a mixing proportion between the toner and the carrier always at a constant value, with the result that the structure of the development apparatus is complex. In addition, since the charging effect of the carrier gradually decreases, over long times it is necessary to replace the carrier periodically. On the other hand, in the single-constituent development apparatus, only the toner is used as a developer agent, and hence there is no need to pay special attention to the mixing proportion control and the periodical replacement of a developer agent. However, in the latter apparatus, a high quality visual image has not to date been obtained, and further the kinds of paper sheets onto which the image is to be transferred were also limited.
On the other hand, an electrostatic recording system employing the wet type development apparatus can provide a high quality visual image. However, since an isoper solution which is a petroleum series solution is employed, careful attention must be paid to its handling and it is necessary to fully ventilate the room in which the system is located. Furthermore, according to this system, it is difficult to obtain a high density developed image, and available paper sheets are limited to only those having a good absorption capability.
In order to overcome the disadvantages of the above-mentioned development apparatuses in the prior art, a wet type development method employing a water-soluble developer agent or an organic liquid developer agent has been proposed in U.S. Pat. No. 4,202,913. According to the proposed development process, a drum of photo-sensitive material and a developer roller submerged in an ink tank are disposed in an opposed close relation without making contact with each other. The ink is formed in a film state on a surface of the developer roller by rotating it. Development is effected such that the electric charge of the latent electrostatic image on the drum attracts the film ink onto the surface of the roller while rotating the drum and the roller in the opposite directions to each other. The liquid developer located in the ink tank is carried up to the development zone in a process dependent upon the liquid developer's viscosity, surface tension and affinity with the developer roller surface. Consequently, it is difficult to maintain uniform thickness of the liquid developer in the development zone on the developer roller surface. Accordingly, the amount of the liquid developer attracted by the electric charge of the latent electrostatic image formed on the drum surface varies depending upon the film thickness of the liquid developer in the development zone, and consequently the latent electrostatic image cannot be developed uniformly and unevenness of development occurs.
Furthermore, the amount of the liquid developer attracted and separated from the developer roller surface does not exactly correlate to the electric attractive force of the latent electrostatic image due to the viscosity, surface tension, etc. of the liquid developer. In other words, sometimes development of the latent electrostatic image will be partially missing, or the liquid developer also adheres onto areas extending beyond that of the latent electrostatic image. Accordingly, a latent electrostatic image can not be developed precisely, and as a result, it is impossible to enhance the resolution of an printed image.